Sooner or later you hit an upload box that says “maximum file size 100 KB” and your photo is 3 MB. It’s one of the most common image problems there is, and it’s also one of the easiest to fix once you know which dial to turn first.
File size comes from three things
An image’s file size isn’t one number you can just type in. It’s the result of three things working together: how many pixels the image has, which format it’s saved in, and how hard that format is compressing the data. To hit a target like 100 KB, you adjust those in a sensible order rather than randomly dragging a quality slider and hoping.
Step 1: Shrink the dimensions first
This is where the biggest savings are. A photo straight off a modern phone is often 4000 pixels wide or more. If the image is only going to be shown at 800 pixels wide, the extra pixels are pure dead weight. Cutting an image from 4000 px to 1000 px wide removes about 94% of its pixels, which usually shrinks the file dramatically before you’ve touched quality at all.
Use a tool that keeps the aspect ratio locked so nothing gets stretched. If you haven’t done this before, our guide on resizing without losing quality covers the basics.
Step 2: Pick the right format
Photographs should almost always be JPG or WebP. If your photo is currently a PNG, converting it to JPG can cut the file to a fraction of its size, because PNG stores every pixel without throwing anything away. PNG only makes sense for screenshots, logos and graphics with flat colors. We compare the three in detail in JPG vs PNG vs WebP.
Step 3: Lower the quality in small steps
Now bring in the quality setting. Don’t jump straight to 20%. Start around 80, check the file size, and step down by five or ten until you’re under your limit. For most photos, quality in the 70 to 85 range looks nearly identical to the original at normal viewing size, while the file is far smaller. Below about 50 you’ll start to see blocky patches around edges and in skies.
What to expect at different sizes
These are rough ranges, not promises, because content matters a lot. A photo of a plain wall compresses much further than a photo of a crowded street.
- Around 600 x 800 px JPG at quality 75-80: often lands somewhere between 40 and 100 KB
- Around 1200 x 800 px JPG at quality 75-80: commonly 100 to 250 KB
- Same image saved as PNG: frequently several times larger than the JPG
When you’re still over the limit
If the file is still too big after dropping the quality to a sensible level, go back to step 1 and reduce the dimensions a bit more. Shaving 10% off the width is usually less noticeable than pushing quality too low. Also worth checking: whether the image is a screenshot with lots of fine text, since those compress poorly as JPG and may look better as a smaller PNG instead.
Frequently asked questions
Can I get exactly 100 KB?
Not to the byte, and you don’t need to. Aim to land a little under the limit, say 90 to 98 KB, so the upload isn’t rejected by a rounding difference.
Will it look worse?
Slightly, if you compress hard, but at the sizes most forms and websites display images, the difference is hard to see.
What if the form asks for exact pixel dimensions too?
Set the exact width and height first, then handle file size with quality and format. Our guide to photo and signature sizes for online forms walks through that case.
You can do all of this in one pass with our free online image resizer: set your dimensions, choose JPG or WebP, and download the result.